1. Field of the Invention
The present invention relates to a printing system and more particularly relates to method and apparatus for formatting characters to be printed to control the actuation of an on/off print mechanism.
2. Prior Art
In the field of data communications various techniques are known for providing a hard copy record of information displayed on a CRT screen or the like or for printing the contents of a data processing memory unit. The most familiar yet perhaps not the most efficient hard copy printers are those using print elements configured in the shape of the characters to be printed. The traditional typewriter as well as higher speed electronic typewriters fall in this category of hard copy printers.
The speed with which data processing systems handle data has dictated that other type printers evolve which are capable of printing hard copy information at speeds not obtainable by the traditional typewriter and its electronically enhanced successors. To achieve greater printing speeds so called dot matrix printers are available which format characters from uniquely configured sequences of dots a line at a time. The increase in speed with which such printers can print is achieved at a cost of perceived degradation in print quality. The typical dot matrix printer is perceived as being useful for rough draft word processing but not of a good enough quality for most office environment printing.
In efforts to overcome the perceived difficulties with traditional typewriter and dot matrix printers, a group of non-impact yet high resolution character forming generators have evolved examples of which include a laser printer and an ink jet printer. The theory of operation for these two types of printers is similar. In the laser printer, for example, a charged photoconductive member is selectively discharged by a laser beam to encode the photoconductive member with information. Once the laser has selectively discharged a member, that member is developed and the developed image transferred to plain paper for subsequent fusing. In the ink jet printer, the development and fusing steps are obviated since the ink jet printer throws ink jet droplets towards a print medium to encode that medium with an information representation.
Some technique must be included in a print system utilizing either a laser printer or an ink jet printer to translate or convert digital data representing the characters to be printed into control signals for selectively activating either the laser in the laser printer or the ink jet generator in the ink jet printer. As it is known, one mechanism for representing characters in a digital format is the so called ASCII format of encoding information. The ASCII system involves the generation of a bit pattern of zeros and ones for each character in a particular language. This bit pattern in turn can be stored in a memory of a digital computer and used to generate control signals for the particular printer incorporated in the data communication's system. In the modern electronic typewriter, for example, the typewriter can interpret ASCII character codes directly to make a determination what character is to strike the print medium.
In the laser printer and/or the ink jet printer such a straightforward interpretation technique is not possible since the laser and/or ink generator must be instructed to throw droplets to form an ink pattern rather than initialize the actuation of a particular character on a print head. In a laser printer, for example, the printer must be selectively turned on and off at controlled locations as the laser beam scans across the photoreceptor or photoconductor width so that only specified portions of the photoconductor member are discharged. The controlled activation is performed line by line as the laser beam sweeps across the photoconductor so that a series of on and off signals must be generated for each print pattern stored in the digital ASCII format. The resolution of a typical laser printer requires that only a small segment or swath of information is encoded onto the photoconductive member each time the laser beam sweeps across the member. It should be readily apparent, therefore, that the laser printer must include a mechanism typically incorporated into electronic circuitry for receiving an ASCII (or similar) bit representation designating a particular character to be printed and converting that ASCII representation into a series of on/off laser controls which will selectively discharge the charged photoconductive member at appropriate intervals as the laser beam sweeps that member.
U.S. Pat. No. 4,059,833 to Kitamura et al and U.S. Pat. No. 4,071,909 to Geller disclose prior art laser printing apparatus including circuitry for converting signals representing the characters to be printed into on/off control signals for selectively dissipating a charge on a charge photoconductive element. Each of the two printing systems disclosed in those patents includes apparatus for storing character representations in an ASCII format. As it is known, an 8 bit storage register is adequate for storing a given alphanumeric character in the ASCII representation system. Both systems utilize circuitry for converting ASCII representation stored in memory into control signals for selectively turning on and off the laser in the laser printing system.
The U.S. Pat. No. 4,059,833 discloses at column 30 a sequence of data transfer steps for parallel loading a bit pattern corresponding to a character in the ASCII system into a parallel to serial shift register and for clocking control signals from this shift register to the laser printer so that the ASCII characters can be encoded onto the photoreceptor. The method disclosed in this patent, however, requires that each character be loaded into the parallel to serial shift register a multiple number of times as that character is printed. Review of this patent would indicate that this technique for control signal generation is inefficient from a data manipulation standpoint.
The U.S. Pat. No. 4,071,909 also discloses a data manipulation technique for generating control signals. According to the technique disclosed in this patent a full page representation must be stored in memory prior to the generation of control signals. In particular, the control technique causes the laser to scan in a direction perpendicular to the normal left to right character appearance common in western cultures. Thus, before the printer can begin formating the data into bit patterns to control on/off laser operation, the entire page to be printed must be stored by the print system so that the characters can be formated.
Both systems disclosed in these patents involve fairly complex data manipulation techniques to perform the data conversion from ASCII coded characters to on/off type control signals to control a laser printer. It is believed that even if the systems embodied by these two patents were updated with present state-of-the-art microprocessor technology, the data manipulation techniques embodied by the hard wire circuitry disclosed in those patents would be inefficient.